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<li><a href="/go/01%E5%BC%80%E5%8F%91%E7%8E%AF%E5%A2%83%E6%90%AD%E5%BB%BA">0.1 开发环境搭建</a></li>
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<li><a href="/go/03%E5%9F%BA%E6%9C%AC%E6%95%B0%E6%8D%AE%E7%B1%BB%E5%9E%8B">0.3 基本数据类型</a></li>
<li><a href="/go/04%E6%B5%81%E7%A8%8B%E6%8E%A7%E5%88%B6%E5%92%8C%E8%BF%90%E7%AE%97%E7%AC%A6">0.4 流程控制和运算符</a></li>
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<li><a href="/go/%E5%B9%B6%E5%8F%91%E7%BC%96%E7%A8%8B/GPM%E5%8E%9F%E7%90%86%E4%B8%8E%E8%B0%83%E5%BA%A6">1.7 GPM与调度分析</a></li>
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  <h1 id="指针类型">指针类型</h1>
<p>任何程序数据载入内存后，在内存都有内存的地址这就是指针。而为了保存一个数据在内存中的地址，就需要指针变量。一个指针的值是一个变量的地址。一个指针对应变量在内存中的存储位置。</p>
<p>例如：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">x := <span class="number">1</span>          <span class="comment">// 声明int类型变量x</span></span><br><span class="line">p := &amp;x         <span class="comment">// &amp;x用于获取变量x的内存地址，返回一个指向x的指针p</span></span><br><span class="line">fmt.Println(*p) <span class="comment">// *p用户获取指针p指向变量的值</span></span><br><span class="line">*p = <span class="number">2</span>          <span class="comment">// 可以重新给*p指针赋值</span></span><br><span class="line">fmt.Println(x)  <span class="comment">// &quot;2&quot;</span></span><br></pre></td></tr></table></figure>
<p>输出：</p>
<figure class="highlight shell"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">1      </span><br><span class="line">2    </span><br></pre></td></tr></table></figure>
<p>用 <code>var x int</code> 声明语句声明一个变量，<code>&amp;x</code> 表达式（取x变量的内存地址）将产生一个指向该整数变量的指针，指针对应的数据类型是 <code>*int</code>，名字为 <code>p</code> ，可以理解为：“p指针指向变量x”或者“p指针保存了x变量的内存地址”。同时 <code>*p</code> 表达式对应p指针指向的变量的值。一般 <code>*p</code> 表达式读取指针指向的变量的值，这里为int类型的值，同时因为 <code>*p</code> 对应一个变量，所以该表达式也可以出现在赋值语句的左边，表示更新指针所指向的变量的值。</p>
<blockquote>
<p>注意：因为指针包含了一个变量的地址，因此如果将指针作为参数调用函数，那将可以在函数中通过该指针来更新变量的值</p>
</blockquote>
<ul>
<li><code>&amp;x</code>：获取变量x的内存地址，返回一个指向x的指针</li>
<li><code>*p</code>：获取指针p指向变量的值</li>
</ul>
<p>看一下 <code>b := &amp;a</code> 的图示：</p>
<p><img src="https://www.liwenzhou.com/images/Go/pointer/ptr.png" alt="取变量地址图示"></p>
<p><strong>总结：</strong> 取地址操作符 <code>&amp;</code> 和取值操作符 <code>*</code> 是一对互补操作符，<code>&amp; </code>取出地址，<code>* </code>根据地址取出地址指向的值。</p>
<p>变量、指针地址、指针变量、取地址、取值的相互关系和特性如下：</p>
<ul>
<li>对变量进行取地址 <code>&amp;</code> 操作，可以获得这个变量的指针变量</li>
<li>指针变量的值是指针地址</li>
<li>对指针变量进行取值 <code>*</code>操作，可以获得指针变量指向的原变量的值</li>
</ul>
<p>指针传值</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">modify1</span><span class="params">(x <span class="type">int</span>)</span></span> &#123;</span><br><span class="line">	x = <span class="number">100</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">modify2</span><span class="params">(x *<span class="type">int</span>)</span></span> &#123;</span><br><span class="line">	*x = <span class="number">100</span></span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> &#123;</span><br><span class="line">	a := <span class="number">10</span></span><br><span class="line">	modify1(a)</span><br><span class="line">	fmt.Println(a) <span class="comment">// 10</span></span><br><span class="line">	modify2(&amp;a)</span><br><span class="line">	fmt.Println(a) <span class="comment">// 100</span></span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<h1 id="new和make">new和make</h1>
<p>先来看一个例子：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> &#123;</span><br><span class="line">	<span class="keyword">var</span> a *<span class="type">int</span></span><br><span class="line">	*a = <span class="number">100</span></span><br><span class="line">	fmt.Println(*a)</span><br><span class="line"></span><br><span class="line">	<span class="keyword">var</span> b <span class="keyword">map</span>[<span class="type">string</span>]<span class="type">int</span></span><br><span class="line">	b[<span class="string">&quot;wangpengliang&quot;</span>] = <span class="number">100</span></span><br><span class="line">	fmt.Println(b)</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>执行上面的代码会引发 <code>panic</code>，为什么呢？ 在Go语言中<strong>对于引用类型的变量，在使用的时候不仅要声明，还要为它分配内存空间，否则值就没办法存储。而对于值类型的声明不需要分配内存空间，是因为它们在声明的时候已经默认分配好内存空间</strong>。Go语言中 <code>new</code> 和 <code>make</code> 是内建的两个函数，主要用来分配内存。</p>
<p>这里 <code>panic</code>原因在于：</p>
<p><code>var a *int</code>  只是声明了一个指针变量 <code>a</code> 但是没有初始化，指针作为引用类型需要初始化后才会拥有内存空间，才可以赋值。比如：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a *<span class="type">int</span></span><br><span class="line">a = <span class="built_in">new</span>(<span class="type">int</span>)</span><br></pre></td></tr></table></figure>
<p><code>var b map[string]int</code> 只是声明变量 <code>b</code> 是一个 <code>map</code> 类型变量而未初始化。需要初始化后才可以赋值，比如：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> b <span class="keyword">map</span>[<span class="type">string</span>]<span class="type">int</span></span><br><span class="line">b = <span class="built_in">make</span>(<span class="keyword">map</span>[<span class="type">string</span>]<span class="type">int</span>, <span class="number">10</span>)</span><br></pre></td></tr></table></figure>
<h2 id="new">new</h2>
<p><code>new</code> 是一个内置的函数，函数签名如下：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">new</span><span class="params">(Type)</span></span> *Type</span><br></pre></td></tr></table></figure>
<p>其中，</p>
<ul>
<li><code>Type</code> 表示类型，<code>new</code> 函数只接受一个参数，这个参数是一个类型</li>
<li><code>*Type</code>表示类型指针，<code>new</code> 函数返回一个指向该类型内存地址的指针</li>
</ul>
<p>使用 <code>new</code> 函数得到的是一个类型的指针，并且该指针对应的值为该类型的零值。比如：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">main</span><span class="params">()</span></span> &#123;</span><br><span class="line">	a := <span class="built_in">new</span>(<span class="type">int</span>)</span><br><span class="line">	b := <span class="built_in">new</span>(<span class="type">bool</span>)</span><br><span class="line">	fmt.Printf(<span class="string">&quot;%T\n&quot;</span>, a) <span class="comment">// *int</span></span><br><span class="line">	fmt.Printf(<span class="string">&quot;%T\n&quot;</span>, b) <span class="comment">// *bool</span></span><br><span class="line">	fmt.Println(*a)       <span class="comment">// 0</span></span><br><span class="line">	fmt.Println(*b)       <span class="comment">// false</span></span><br><span class="line">&#125;	</span><br></pre></td></tr></table></figure>
<h2 id="make">make</h2>
<p><code>make</code> 也是用于内存分配的，与 <code>new </code>区别在于它只用于 <code>slice</code>、<code>map</code>以及 <code>channel</code> 的内存创建，而且它返回的类型就是这三个类型本身，而不是他们的指针类型。</p>
<blockquote>
<p>因为这三种类型就是引用类型，所以就没有必要返回他们的指针了</p>
</blockquote>
<p>函数签名如下：</p>
<figure class="highlight go"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">func</span> <span class="title">make</span><span class="params">(t Type, size ...IntegerType)</span></span> Type</span><br></pre></td></tr></table></figure>
<h1 id="new与make的区别">new与make的区别</h1>
<ol>
<li>二者都是用来做内存分配的。</li>
<li><code>make</code> 只用于<code>slice</code>、<code>map</code> 、<code>channel</code> 的初始化，返回这三个引用类型本身</li>
<li><code>new</code> 用于类型的内存分配，并且内存对应的值为类型零值，返回的是指向类型的指针</li>
</ol>

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